1. Field of the Invention
This invention is concerned with analysis of semiconductor processing solutions, particularly with determination of the silicon, germanium or titanium concentration in alkaline etchant solutions.
2. Description of the Related Art
Alkaline etchant solutions are widely-used in important industrial processes to provide anisotropic (pyramidal) etching of single-crystal silicon needed for fabrication of high-speed integrated circuit (IC) chips with copper circuitry, efficient solar cells based on nano-technology, and micro-electro-mechanical systems (MEMS) devices, such as accelerometers used in automotive air bag systems. To provide the precise nano-scale silicon features required for such applications, it is necessary to precisely control the silicon etch rate, which tends to decrease as silicon etchant products build up in the etchant solution. A means of accurately measuring the concentration of silicon etchant products in alkaline etchant solutions is needed to allow the etch time to be adjusted as the etch rate changes, and the etchant solution to be replaced based on need rather than a schedule. Conventional methods for detecting silicon ions in solution based on near-infrared (NIR) spectroscopy or ion chromatography (IC) do not provide the sensitivity needed.
Alkaline etchant solutions are also used to etch other materials in industrial manufacturing processes. For example, germanium is increasingly used as a replacement for silicon in semiconductor devices and an improved method of measuring the concentration of germanium ions in alkaline etchant solutions is also needed. As a further example, titanium spattered onto the source and drain zones of silicon transistors is heated to form TiSi, which provides an ohmic electrical contact to the doped silicon in these zones. An alkaline etchant is used to remove unreacted titanium from the transistor. A suitable method for detecting titanium ions in alkaline etchant solutions is also needed.
U.S. Pat. No. 8,008,087 to Shalyt et al. describes a method for detecting low concentrations of silicon ions in a semiconductor etchant solution by adding a predetermined concentration of fluoride ions to a test solution comprising a predetermined volume of the etchant solution, and measuring the concentration of fluoride ions in the test solution. Reaction with silicon ions in the test solution reduced the concentration of fluoride ions, which were present in stoichiometric excess, so that the silicon concentration of the etchant solution could be calculated from the difference between the predetermined and measured concentrations of fluoride ions in the test solution. This prior art method was successfully applied to analysis of silicon ions in silicon nitride etchants comprising a high concentration of phosphoric acid but cannot be applied directly for analysis of alkaline etchant solutions since fluoride does not react with silicon ions in alkaline solutions.